Having given this some more thought, I suspect that the lang attribute in the WOFF spec may not actually be equivalent to the xml:lang attribute. The xml:lang attribute really says "this is the language of the text in this element". The woff lang attribute may be saying that by default in many cases, but is actually saying "this is the element to use for a given user". These things could be very different if, say, you had to use slightly different licensing text for Hungarian users but you didn't have a means to translate your license text into Hungarian. [1]
In that case, I think it would be better to rename this attribute. Perhaps locale would be a better name. (The values would remain the same.)
NB: We then have an additional discussion about whether xml:lang ought to be supported by the schema (even though it is an attribute defined by the XML spec, it still needs to be defined in the schema when used). xml:lang would be used by things like spell-checkers, voice browsers, etc that need to understand the language of the text they are processing.
RI
[1] For more on this kind of distinction, see http://www.w3.org/International/questions/qa-when-xmllang xml:lang in XML document schemas.
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Richard Ishida
Internationalization Lead
W3C (World Wide Web Consortium)
http://www.w3.org/International/http://rishida.net/